Methods: Motor evoked potentials (MEPs) were recorded bilaterally from tibialis anterior (TA) and plantar foot muscles (PF) after TCES (train of 5 stimuli). MEP peak-to-peak amplitude was determined from the average of 3 superimposed MEPs from each muscle. Maximum M-potentials were recorded from TA and PF after supramaximal stimulation of the peroneal nerve at the knee and the posterior tibial nerve at the ankle. The peak-to-peak MEP amplitude was divided by the maximum M-potential amplitude then multiplied by 100 to determine the percent of maximum M-potential represented by the MEP.

Results: MEPs and maximum M-potentials were recorded from 5 TA muscles and 5 PF muscles in 3 anaesthetized patients undergoing cervical or thoracic spine surgery. The mean MEP amplitude from TA and PF was 0.136 ± 0.41 mV and 0.977 ± 0.482 mV respectively. The mean maximum M-potential amplitude from TA and PF was 5.3 ± 1.8 mV and 13.1 ± 5 mV respectively. The mean percent of maximum M-potential evoked after TCES was 2.6 ± 0.5 % for TA, and 7.1 ± 2.2 % for PF.

Conclusion: TCES evokes only a small percentage of the total membrane current available in TA and PF muscles. This limitation should be considered when interpreting neurogenic and myogenic MEPs during nerve surgery.